1. Field of the Invention
The present invention belongs to a technological field related to magnetic resonance imaging systems (MRI systems) utilized for imaging diagnosis of living beings, and particularly relates to a magnet apparatus for forming a uniform static-magnetic-field region.
2. Description of the Related Art
Magnetic resonance imaging systems utilizing a magnet apparatus are grossly divided, according to the form of their cryostats—which incorporate magnetic-field generating means such as superconductive coils—into facing-type systems, in which a spherical and uniform static-magnetic-field region is formed between a pair of cryostats, and cylindrical-type systems. Recently, the facing-type systems, which are better from viewpoints such as the subject's feeling of not being confined, and convenience in diagnosing practitioners' accessing the subject, are becoming the mainstream. The magnetic-field strength in the spherical and uniform static-magnetic-field region in the facing-type magnetic resonance imaging system is generally around 6000 through 10000 gauss, with the margin for error usually being several ppm. Meanwhile, in order to achieve uniformity of the static magnetic field, a plurality of magnetic-field generating means are ordinarily used in combination. In addition, in order to reduce fringe field to the outside of the magnet apparatus, the system includes a magnetic-shielding means.
Facing-type magnetic resonance imaging systems as described above include vertical-type systems, in which a pair of magnetic-field generating means are arranged vertically (e.g., see Patent Document 1: Japanese Laid-Open Patent Publication 153408/1997), and horizontal-type systems, in which a pair of magnetic-field generating means are arranged horizontally (e.g., see Patent Document 2: Japanese Laid-Open Patent Publication 187439/1997). For the magnetic-field generating means, superconductive coils are generally used.
In Patent Document 1, as a measure against fringe field to the outside of the magnet apparatus, shield coils having approximately the same diameter as the diameter of main coils are installed with their center axes being identical to those of the main coils, and the shield coils are positioned, along the center axis, farther away from the center of the uniform static magnetic field than the main coils. Current flows through the shield coils in a direction opposite to the main-coil current direction, to reduce fringe field.
Meanwhile, in Patent Document 2, shield coils having a larger diameter than main coils are disposed to reduce fringe field. In addition, it is disclosed that resistive shield coils are installed.
In the magnetic resonance imaging system disclosed in Patent Document 1, so as not to lower the magnetic field strength in the uniform static-magnetic-field region due to the shield coils, the shield coils are kept as far away from the main coils as possible. More specifically, the separation of the shield coils, along the center axis of both, from the center of the uniform static-magnetic-field region is made large. Accordingly, there has been a problem in that the fringe field spreads along the axis around which the coil arrangement is centered, due to the magnetic field generated from the shield coils.
In the magnetic resonance imaging system disclosed in Patent Document 2, shield coils that are larger in radius than the main coils are disposed around the main coils. Meanwhile, when the radius of a circular coil is R and the current is I, the magnetic field H along the center axis of the coil at the point where the distance from the center of the coil is X is expressed according to the following equation:H=0.5×I×R2/(R2+X2)1.5
Briefly, there has been a problem in that, due to the shield coils having a large radius, the fringe field spreads along the center axis of the two coils likewise as in Patent Document 1.